Which part of the hand should be used to work a shampoo into a lather?

Using the cushions of the fingertips is the most effective way to work a shampoo into a lather. The cushions of the fingertips provide a gentle yet effective method of applying pressure to the scalp without causing discomfort. This area allows for a controlled and precise application, ensuring that the shampoo is evenly distributed throughout the hair and scalp.

Additionally, using the cushions helps in stimulating the scalp, which can enhance circulation and promote a healthy environment for hair growth. Fingertips also allow for better maneuvering and access to different areas of the scalp, making it easier to create a rich lather.

In contrast, using the palm may not provide the same degree of precision or stimulation to the scalp, while using the fingers alone might not harness the optimal pressure needed for effective lathering. Relying on the knuckles could risk applying too much pressure or creating discomfort for the client. Thus, the cushions of the fingertips offer the best combination of gentle contact and effective lathering action.